1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) and, more particularly, to an LCD having wide vertical and horizontal viewing angles and including a liquid crystal layer that easily makes a transition to an initial bend phase.
2. Description of the Related Art
Nowadays, in order to overcome the shortcomings of conventional display devices such as cathode ray tubes (CRTS) that are -heavy and large-sized, much attention is being paid to flat panel display devices (FPDs), for example, LCDs, organic light emitting display devices (OLEDs), and plasma display panels (PDPs).
In a typical LCD, liquid crystals are injected between two substrates including electrodes for generating an electric field. The electric field is generated between the substrates by the application of different electric potentials to the electrodes, so that the arrangement of liquid crystal (LC) molecules is changed. Thus, optical transmittance is controlled so as to display an image on a screen.
FIGS. 1A and 1B are perspective views illustrating an operating principle of a twisted nematic (TN) mode LCD, which is the most representative type of LCD.
Referring to FIG. 1A, the TN mode LCD includes a first substrate 101, a second substrate 103, a light source (not shown), and a liquid crystal layer 106. A first electrode (not shown) and a first alignment layer (not shown) are formed on one surface of the first substrate 101, and a first polarizer 102 is formed on the other surface thereof. A second electrode (not shown) and a second alignment layer (not shown) are formed on one surface of the second substrate 103, and a second polarizer 104 is formed on the other surface thereof. The light source supplies light 105 from above the other surface of the second substrate 103. The liquid crystal layer 106 is filled between the first and second substrates 101 and 103.
A polarization axis of the first polarizer 102 is located in the same direction as a direction 107 in which the first alignment layer formed on one surface of the first substrate 101 is rubbed. A direction 108 in which the second alignment layer formed on one surface of the second substrate 103 is rubbed is perpendicular to the direction 107 in which the first alignment layer is rubbed. A polarization axis of the second polarizer 104 is located in the same direction as the direction 108 in which the second alignment layer is rubbed.
Referring to FIG. 1A, in an inactivated state where no voltage is applied between the first and second electrodes, it can be seen that the major axes (i.e., local optical axes) of LC molecules of the liquid crystal layer 106 filled between the first and second substrates 101 and 103 are gradually twisted due to the directions 107 and 108 in which the first and second alignment layers are rubbed. Thus, an LC molecule 106a close to one surface of the first substrate 101 is twisted at an angle of 90° to an LC molecule 106b close to one surface of the second substrate 103. As a result, the light 105 emitted from the light source is linearly polarized by the second polarizer 104 (refer to 105a), rotated by the LC molecules of the liquid crystal layer 106 (refer to 105b), and then externally emitted through the first polarizer 102 having the polarization axis perpendicular to that of the second polarizer 104 (refer to 105c).
Referring to FIG. 1B, in an activated state where a voltage is applied between the first and second electrodes, LC molecules of the liquid crystal layer 106 are not twisted any more and become parallel to each other due to an electric field generated by the voltage. Thus, light 105a obtained by linearly polarizing the incident light 105 is not rotated any more and is wholly absorbed in the first polarizer 102.
However, the TN mode LCD has narrow horizontal and vertical viewing angles and a slow response speed. Thus, optically compensated bend (OCB) mode LCDs have been proposed to solve the problems of the TN mode LCD, but it is still necessary to improve horizontal and vertical viewing angles.